FIELD OF THE INVENTION
This invention relates to a semiconductor laser device and more particularly, to a double-hetero structure laser having a structure in which an active layer is curved.
Transverse mode control of semiconductor laser is indispensable for putting the laser into practical application and a wide variety of device structures have been proposed in the past to realize this control. One of these proposals includes a structure in which an active layer is curved. This structure is formed by forming a double-hetero structure on a semiconductor substrate having a V-shaped or trapezoid groove in such a fashion that the active layer is curved. In this case, the thickness of the active layer reaches the maximum at the center of the groove and progressively decreases to the periphery due to a liquid phase epitaxial growth mechanism. Accordingly, the effective refractive index of the active layer becomes maximal at the center and progressively decreases towards the periphery and causes the difference in the effective refractive indices in directions parallel to the junction, thus making it possible to stabilize the laser transverse mode. (See: Articles to be announced at the No. 28th meeting of Society of Applied Physics; p 182, (1981), and articles to be announced at the No. 42nd meeting of Society of Applied Physics; p 171 (1981)). U.S. Pat. No. 3,978,428, for example, discloses semiconductor lasers having such a structure.
However, we have found a problem that in accordance with the structure described above as an example of the prior art, the difference in the effective refractive indices becomes too great between a light emitting region and regions outside the former and hence, a higher order transverse mode is likely to occur even at a low optical output.